Transparent porous SiO2 coating for a transparent substrate material

ABSTRACT

A transparent porous SiO2 coating for a transparent substrate material, for example polycarbonate, has advantageous antireflective properties. The coating is produced by a sol-gel process, at least one component which causes porosity being present during at least part of the sol-gel process and being removed and/or destroyed after the sol-gel process has ended. The at least one component which causes porosity is a polymer, the average molar mass of the polymer being preferably =5000 Da to =50 000 Da, more preferably =10 000 Da to =20 000 Da.

The present invention relates to the field of transparent materials, in particular transparent materials whose reflection has been reduced.

With many transparent materials, in particular transparent materials based on plastic, such as, for example, polycarbonate etc., there arises the difficulty that said materials sometimes have undesirable reflection properties which make their use difficult or even impossible in many applications.

For this reason numerous attempts have been undertaken to make transparent materials less reflective, in particular by application of further layers.

Toward that end it has been proposed, for example, to reduce reflection by means of what is termed a “flower-like alumina” layer (see Yamaguchi et al, Journal of Sol-Gel Science & Technology, 2005, 33, 117-120). However, this approach requires an annealing step at increased temperatures (approx. 400° C).

Other proposed coatings include multilayer systems with varying refractive indices, e.g. multilayer systems consisting of SiO₂ and TiO₂. Here too, however, annealing steps are necessary, mostly at temperatures in excess of 400° C. (see M. Walther, OTTI Seminar Regensburg, September 2005). Other systems use layers consisting of TiO₂ and MgF₂ (see EP 564 134 B1) in which fluorocarbon resins are provided in addition. The disadvantage of this system lies once again in the difficulty of application.

The object of the present invention is therefore to provide a transparent coating for a transparent substrate material which at least partially overcomes the aforementioned disadvantages and in particular is easy to apply.

This object is achieved by means of a transparent coating as claimed in claim 1 as well as by means of the method as claimed in claim 6.

According to the invention a transparent coating for a transparent substrate material is proposed which is characterized in that the coating is based on SiO₂ and has a porosity of ≧35% to ≦65%.

The description “based on SiO₂” within the meaning of the present invention means or includes in particular that the coating contains SiO₂ as its main component. In this case preferably ≧270%, more preferably ≧80% and most preferably ≧90% to ≦100 of the coating consists of SiO₂.

The term “transparent” within the meaning of the present invention means or includes in particular a transmittance of ≧90% in the wavelength range used in the individual case, in particular in the visible wavelength range.

By means of an inventive SiO₂ coating of this kind one of more of the following advantages can be achieved in many applications within the present invention:

-   -   The coating is essentially homogeneous and a single coating is         sufficient for many applications (in contrast to the multilayer         systems cited above).     -   In most applications within the present invention the coating         can—as will be described in the following—be applied by simple         dipping, which means that complicated and time-consuming         application steps, which in particular have to be carried out at         high temperature, can be avoided.     -   As will also be described below, the thickness of the coating         produced is in the range of 50-200 nanometers in most         applications. The coating is therefore largely insensitive         toward thermal and mechanical stress (in particular flexural         stress) and only marginally affects component dimensions and         tolerances.

According to a more preferred embodiment the porosity of the coating amounts to ≧40% to ≦60%, more preferably ≧45% to ≦55%.

A preferred embodiment of the invention is characterized in that the thickness of the coating lies in the range ≧95 nm to ≦135 nm. This has proved particularly favorable for many applications. Preferably the thickness of the coating is ≧100 nm to ≦130 nm.

A preferred embodiment of the invention is characterized in that the refractive index n₁ of the coating lies in the range ≧0.8*√{square root over (n₂)} to ≦1.2*√{square root over (n₂)}, where n₂ is the refractive index of the substrate. In this case the reflection can be reduced even further for many applications within the present invention.

The refractive index n₁ of the coating preferably lies in the range ≧0.9*√{square root over (n₂)} to ≦1.1*√{square root over (n₂)}.

A preferred embodiment of the invention is characterized in that the coating is essentially a porous full-mold body, in particular a homogeneous porous full-mold body, or forms a body of said kind.

The term “essentially” in this context denotes in particular ≧90 vol-%, preferably ≧295 vol-%, of the coating.

By this means it is possible in many applications within the present invention to achieve a coating that is easy to produce and reduces reflections even further.

A preferred embodiment of the invention is characterized in that the coating has transmission-enhancing properties, in particular for light in the visible wavelength range.

Preferably the coating is able to increase the transmission of the substrate by ≧2%, preferably by ≧4%, in the wavelength range used in the individual case, in particular in the visible wavelength range.

A preferred embodiment of the invention is characterized in that the average diameter of the pores ranges from ≧5 nm to ≦50 nm. In many applications of the present invention this has proved particularly favorable in terms of the anti-reflective properties of the coating. Preferably the average diameter of the pores ranges from ≧10 nm to ≦40 nm, more preferably from ≧10 nm to ≦25 nm.

A preferred embodiment of the invention is characterized in that the diameter of ≧90% of the pores ranges from ≧5 nm to ≦50 nm.

A preferred embodiment of the invention is characterized in that the diameter distribution of the pores of the inventive coating essentially follows a log-norm distribution with a half-width of ≦10 nm, preferably ≦8 nm, more preferably ≦5 nm.

“Essentially” in this context means that ≧90% of the pores, preferably ≧95% of the pores, and most preferably ≧98% of the pores follow this distribution.

A distribution of this kind has proved particularly favorable for many applications of the present invention, since an optically particularly homogeneous coating can thus be achieved.

A preferred embodiment of the invention is characterized in that the coating is produced by means of a sol-gel method.

The present invention also relates to a method for producing a transparent coating for a transparent substrate material, characterized in that the method is based on a sol-gel process.

The description “sol-gel process” or “sol-gel method” within the meaning of the present invention means or includes in particular all processes and/or methods in which silicon precursor materials, in particular silicon halogenides and/or silicon alkoxides, are subjected in solution to hydrolysis and subsequent condensation.

A preferred embodiment of the invention is characterized in that during at least a part of the sol-gel process at least one porosity-inducing component is present which will be removed and/or destroyed following termination of the sol-gel process.

A preferred embodiment of the invention is characterized in that the at least one porosity-inducing component is a polymer, the average molecular weight of the polymer preferably being ≧5,000 Da to ≦50,000 Da, more preferably ≧10,000 Da to ≦20,000 Da.

A preferred embodiment of the invention is characterized in that the polymer is an organic polymer, preferably selected from the group containing polyethylene glycol, polypropylene glycol, copolymers of polyethylene glycol and polypropylene glycol, polyvinyl pyrrolidone, polyether, alkyl-, cycloalkyl- and/or aryl-substituted polyether, polyester, alkyl-, cycloalkyl- and/or aryl-substituted polyester, in particular polyhydroxy butyrate or mixtures thereof.

General groups/molecule definition: General groups or molecules, such as e.g. alkyl, alkoxy, aryl etc., are claimed and described within the description and the claims. Unless described otherwise, the following groups are preferably used within the generally described groups/molecules in the scope of the present invention:

Alkyl: linear and branched C1-C8 alkyls

Long-chain alkyls: linear and branched C5-C20 alkyls

Alkenyl: C2-C6 alkenyl

Cycloalkyl: C3-C8 cycloalkyl

Alkoxide/alkoxy: C1-C6 alkoxy, linear and branched

Long-chain alkoxide/alkoxy: linear and branched C5-C20 alkoxy

Aryl: selected from aromatic compounds having a molecular weight below 300 Da

Polyether: selected from the group containing H—(O—CH₂—CH(R))_(n)—OH and H(O—CH₂—CH(R))_(n)—H, where R is independently selected from: hydrogen, alkyl, aryl, halogen, and n from 1 to 250

Substituted polyether: selected from the group containing R₂—(O—CH₂—CH(R₁))_(n)—OR₃ and R2 —(O—CH₂—CH(R₂))_(n)—R₃, wherein R₁, R₂, R₃ is independently selected from: hydrogen, alkyl, long-chain alkyls, aryl, halogen, and n is in the range from 1 to 250

Ether: The compound R₁—O—R₂, where each R₁ and R₂ are independently selected from the group containing hydrogen, halogen, alkyl, cycloalkyl, aryl, long-chain alkyl

Unless stated otherwise, the following groups/molecules are more preferred groups/molecules within the general groups/molecule definition:

Alkyl: linear and branched C1-C6 alkyl,

Alkenyl: C3-C6 alkenyl

Cycloalkyl: C6-C8 cycloalkyl

Alkoxy, alkoxide: C1-C4 alkoxy, in particular isopropyloxide

Long-chain alkoxy: linear and branched C5-C10 alkoxy, preferably linear C6-C8 alkoxy

Polyether: selected from the group containing H—(O—CH₂—CH(R))_(n)—OH and H(O—CH₂—CH(R))_(n)—H, where R is independently selected from: hydrogen, alkyl, aryl, halogen, and n is in the range from 10 to 250.

Substituted polyether: selected from the group containing R₂ —(O—CH₂—CH(R₁))_(n)—OR₃ and R₂ —(O—CH₂—CH(R₂))_(n)—R₃, where R₁, R₂, R₃ is independently selected from: hydrogen, alkyl, long-chain alkyls, aryl, halogen, and n from 10 to 250.

A preferred embodiment of the invention is characterized in that the polymer is washed out following termination of the sol-gel process.

A preferred embodiment of the invention is characterized in that the polymer is washed out following termination of the sol-gel process by means of annealing, in particular at a temperature of ≧80° C. to ≦100° C., preferably with water.

A preferred embodiment of the invention is characterized in that the polymer is burned out following termination of the sol-gel process, in particular at a temperature of ≧250° C.

A preferred embodiment of the invention is characterized in that the silicon is added in the form of a silicon alkoxide precursor solution.

A preferred embodiment of the invention is characterized in that the pH value of the silicon-containing precursor solution is in the range from ≧1 to ≦4.

The invention also relates to a transparent coating for a transparent substrate, produced in accordance with the inventive method.

The invention also relates to an optical component comprising a transparent substrate as well as a coating applied and/or arranged on the substrate according to the present invention.

A preferred embodiment of the invention is characterized in that the substrate is selected from the group containing glass, transparent plastics, preferably selected from the group containing polycarbonate, polyacrylic and mixtures thereof, as well as mixtures thereof.

The present invention also relates to a method for producing an optical component according to the invention, characterized in that the coating is applied to the substrate by dipping and/or spin-coating.

The present invention also relates to the use of an inventive coating and/or an inventive optical component for

-   -   optical instruments     -   spectacles     -   headlight housings in the automotive engineering field     -   windows, particular in the automotive engineering field     -   cockpit windshields

The aforementioned components and the claimed components that are described in the exemplary embodiments and are to be used according to the invention are not subject to any special exception conditions in terms of their size, formal design, choice of material and technical design concept, so the selection criteria known in the field of application can be applied without restriction.

Further details, features and advantages of the subject matter of the invention will emerge from the dependent claims as well as from the following description of the associated drawings, in which—by way of example—an exemplary embodiment of an inventive coating is shown.

FIG. 1 shows a diagram containing two transmission measurements of a polycarbonate substrate coated according to a first embodiment of the invention and an uncoated polycarbonate substrate; and

FIG. 2 is a photograph of a polycarbonate substrate, half of which is coated according to a first embodiment of the invention.

FIGS. 1 and 2 relate to the example I described below:

EXAMPLE I:

A polycarbonate-based optical component was produced as follows:

First, two solutions were prepared:

Solution 1: 4 g polyethylene glycol was prepared in 50 ml ethanol and water was slowly added while stirring until complete dissolution occurred. Next, 4 drops 1N HCl were added.

Solution 2: 4 ml tetraethoxysilane in 20 ml EtOH

Solution 2 was then filled to 50 ml with solution 1 and stirred for 2 h.

The polycarbonate substrate was conditioned by means of flame silanization in order to increase the bonding between substrate and coating.

The polycarbonate substrate was then dip-coated with the solution (rate 50 mm/min). It was then dried in air and annealed in the oven at 100° C. for 2 h. After cooling down it was stored in water for 1 min.

The porosity of the coating in this case was 55%.

FIG. 1 shows a diagram containing two transmission measurements of the polycarbonate substrate coated according to example I and the uncoated polycarbonate substrate. An improvement in transmission by approx. 5% can be seen.

FIG. 2 shows a photograph of a polycarbonate substrate, half of which was coated according to example I. It can clearly be seen that reflection has been significantly reduced. 

1. A transparent coating for a transparent substrate material, wherein the coating is based on SiO₂ and has a porosity of ≧35% to ≦65%.
 2. The coating according to claim 1, wherein the thickness of the coating lies in the range from ≧95 nm to ≦135 nm.
 3. The coating according to claim 1, wherein the refractive index n₁ of the coating lies in the range from ≧0.8*√{square root over (n₂)} to ≦1.2*√{square root over (n₂)}, where n₂ is the refractive index of the substrate.
 4. The coating according to claim 1, wherein the coating is a porous full-mold body.
 5. The coating according to claim 1, wherein the SiO₂ coating is produced by means of a sol-gel method.
 6. A method for producing a transparent coating for a transparent substrate material, comprising the step of basing the method on a sol-gel process.
 7. The method according to claim 6, wherein during at least a part of the sol-gel process at least one porosity-inducing component is present which will be at least one of removed and destroyed following termination of the sol-gel process.
 8. The method according to claim 6, wherein the at least one porosity-inducing component is a polymer, the average molecular weight of the polymers being ≧5,000 Da to ≦50,000 Da.
 9. The method according to claim 6, wherein the polymer is an organic polymer selected from the group consisting of polyethylene glycol, polypropylene glycol, copolymers of polyethylene glycol and polypropylene glycol, polyvinyl pyrrolidone, polyether, alkyl-, cycloalkyl- and/or aryl-substituted polyether, polyester, alkyl-, cycloalkyl- and/or aryl-substituted polyester or mixtures thereof.
 10. The method according to claim 6, wherein the polymer is washed out following termination of the sol-gel process.
 11. The method according to claim 10, wherein the polymer is washed out by means of annealing following termination of the sol-gel process.
 12. The method according to claim 6, wherein the silicon is added in the form of a silicon alkoxide precursor solution.
 13. The method according to claim 6, wherein the pH value of the silicon-containing precursor solution is in the range from ≧1 to ≦4.
 14. A transparent coating for a transparent substrate, produced according to claim
 6. 15. An optical component including a transparent substrate as well as a coating which is at least one of applied and arranged on the substrate according to claim
 1. 16. The optical component according to claim 15, wherein the substrate is selected from the group containing glass, transparent plastics, selected from the group containing polycarbonate, polyacrylic and mixtures thereof, as well as mixtures thereof.
 17. A method for producing an optical component according to claim 15, wherein the coating is applied to the substrate by at least one of dipping and spin-coating.
 18. A method comprising the step of using of a coating being based on SiO₂ and having a porosity of ≧35% to ≦65% for an optical component including a transparent substrate as well as the coating which is at least one of applied and arranged on the substrate for one of the following selected from the group consisting of: optical instruments, spectacles, headlight housings in the automotive engineering field windows or in the automotive engineering field, and cockpit windshields. 